This invention relates to a knocking control system for internal combustion engines, and particularly to a system of this kind for an internal combustion engine equipped with a device which varies the opening and closing timing or lift of inlet valves and exhaust valves of the engine depending on operating conditions of the engine in order to prevent knocking.
An internal combustion engine is conventionally known, e.g. from Japanese Patent Publication (Kokoku) No. 49-33289, which is capable of changing the valve timing (valve opening period or valve lift) of inlet valves and/or exhaust valves between low speed valve timing suitable for a lower engine rotational speed region and high speed valve timing suitable for a higher engine rotational speed region, in order to enhance the charging efficiency or combustion efficiency.
Further, a control method for internal combustion engines is known, e.g. from Japanese Patent Publication (Kokoku) 57-30980, in which knocking occurring in the engine is detected, and the ignition timing is retarded upon detection of knocking to thereby eliminate the knocking.
However, if knocking control is effected in an internal combustion engine equipped with the valve timing device, there occur the following problems:
Knocking is caused by abnormal combustion within engine cylinderds. The magnitude (knocking noise level) and frequency of knocking, as well as the crank angle at which knocking occurs vary if the valve timing and/or lift of the inlet and exhaust valves and/or the compression ratio vary. Further, the noise level (background level) of the output of a knocking sensor, on the basis of which a knocking discrimination level is set, varies at the time of changeover of the valve timing when changeover of rocker arms and oil passages, etc. is effected. Also, the ignition timing at which knocking occurs and the optimal ignition timing (MBT) are different between the low speed valve timing and the high speed valve timing. Therefore, knocking is not eliminated to thereby cause damage to the engine and degraded driveability and hence marketability of same, unless the retarding amount, retarding speed, advancing amount, and advancing speed of the ignition timing are set to respective appropriate values depending on the actual valve timing when the ignition timing is retarded upon detection of knocking to eliminate same and when it is advanced after elimination of the knocking.
According to the above described conventional method, the knocking discrimination level is calculated by the use of the same equation over the entire engine rotational speed range, on the basis of the amplitude of vibration (noise level) of the engine detected by a noise gate, which is operable in a crank angle region in which knocking does not occur, as shown in FIG. 19. Then, it is determined that knocking has occurred, when the level of vibration detected by a knocking gate, which is operable in a crank angle region in which there is a possiblity of occurrence of knocking, exceeds the calculated knocking discrimination level.
However, the noise level detected by the noise gate has an inclination to increase as the engine rotational speed increases. Further, the noise level at the low speed valve timing is different from that at the high speed valve timing such that the former is higher than the latter. This is because if the low speed valve timing is selected in the higher engine rotational speed region, the gradient of the lift of a cam for driving the inlet or exhaust valve with respect to crank angle becomes very sharp and the load exerted by a rocker arm spring on a rocker arm in sliding contact with the cam becomes large, so that vibration caused by the mutually sliding cam and rocker arm increases.
Therefore, if both the knocking discrimination levels at the low and high speed valve timings are calculated by the use of the same equation as in the conventional method, the calculated results become greatly different from each other due to the difference in the noise level between the low and high speed valve timings. Further, since the noise level abruptly increases when the engine rotational speed rises above e.g. about 4800 rpm, and the increasing rate of the noise level at the low speed valve timing is larger than that at the high speed valve timing, the knocking discrimination level at the low speed valve timing in the higher engine rotational speed region becomes considerably higher than that at the high speed valve timing in the same region. Therefore, knocking, even if it occurs, cannot be positively detected at the low speed valve timing in the higher engine rotational speed region.